In resistance welding, there are four primary factors involved in making a good weld. These factors are current, pressure applied, time duration of welding power application, and electrical contact area with the workpiece. Resistance welding joins the workpieces at the faying or contacting surfaces with the heat obtained from workpiece resistance to the flow of electric current through an electric circuit which includes at least one electrode. The current applied through the workpieces is usually in the thousands of amperes, and may be as much as 100,000 amperes. One electrode acts as the current source, while the ground may be through another electrode or through a fixture that holds the workpiece.
Conventional welding systems that do not monitor weld quality in real-time must utilize empirical control methods for the welding process. As the electrode deteriorates due to the forming of many spot welds, the electrode tip contact area increases. This causes the current per unit area to decrease thereby increasing the time and current required to form a good weld, and if uncompensated results in an underdeveloped weld nugget. Further, deteriorating electrode conditions affect the heat loss that occurs during the weld time. As a result, the conditions for forming a spot weld are continuously changing from weld to weld, requiring some type of compensation in order to produce quality welded parts.
One method that has traditionally been used to compensate for these deteriorating conditions is to form extra spot welds to assure structural integrity. This becomes very costly when mass producing parts, and is therefore an undesirable solution. In many situations, such as weld nut and balance weight welding, the part is too small to have redundant welds and a bad weld results in a product failure.
Another method that has been traditionally employed is a stepper method. The percent heat applied during the weld time is increased in steps after predetermined numbers of welds are formed. The electrodes are replaced after the percent heat has been increased to a predetermined maximum percent heat, and the allowed number of welds for that step have been made.
An existing real-time welding system is described in U.S. Pat. No. 4,542,277, issued to Dimitrios G. Cecil, which is hereby incorporated by reference in its entirety. In the existing system, an externally mounted transducer such as a linear variable differential transformer (LVDT) is mounted to the side of the cylinder shell. The LVDT core moves with the cylinder electrode in side-by-side spaced apart parallel relationship. Because a conventional LVDT has a length to sensing range ratio of about two to one, the LVDT must be calibrated so that the fit up position of the parts to be welded is in the linear region of the LVDT output. Side mounting the LVDT to the cylinder facilitates LVDT calibration, to the linear region because the LVDT can be re-positioned without much difficulty. Accordingly, the LVDT may be appropriately positioned based on the thickness of the workpieces to be welded so that the LVDT output is in its linear region at electrode fit up.
Although welding systems with externally mounted sensors (including LVDTs) have been used in many applications that have been successful, many times in the industrial workplace there are space constraints for mounting the welding gun assembly. In particular, it may be necessary to mount the assembly adjacent walls and other machinery. The externally mounted sensor restricts the available mounting arrangements for the cylinder. Still further, the external sensor is subject to harsh environmental conditions. Further, contact with an operator or anything else in the workplace area may potentially subject the sensor to miscalibration.